Most modern electronic equipment requires a power supply to provide a direct current (DC) operating potential to the electronic components contained therein. Common types of electronic equipment which use power supplies include personal computers, energy systems, telecommunication systems, audio-video equipment, consumer electronics, automotive components, and other devices which utilize integrated circuits, semiconductor chips, or otherwise require DC operating potential. Most, if not all, semiconductor components require a low voltage DC operating potential. However, many sources of electric power are alternating current (AC), or high voltage DC, which must be converted to low voltage DC for the electronic equipment.
In one common arrangement, the AC/DC power supply receives an AC input voltage, e.g., between 110 and 240 VAC, and converts the AC input voltage to the DC operating voltage. The AC voltage is routed through a full-wave rectifier bridge and filtered to produce a high voltage DC signal. The high voltage DC signal is processed through a pulse width modulated (PWM) controller and transformer assembly to generate the low voltage, regulated DC output voltage, which is used as the operating potential for the semiconductor components and other devices requiring low voltage DC supply in the electronic equipment. The low voltage DC signal is typically in the range of 1 to 12 VDC. In other cases, a DC/DC power supply receives a high voltage DC signal and provides the low voltage DC signal necessary for the electronic equipment.
Power supplies which include a PWM controller use a bias voltage to power the integrated circuit (IC) PWM controller. The bias voltage can be obtained by using a separate, secondary winding as part of the transformer assembly. The bias voltage is obtained from the secondary winding. The bias voltage is then rectified. The bias voltage has a certain degree of variation which depends upon the input voltage supplied to the transformer assembly. A common approach to control the level of the bias voltage is to adjust the turns ratio of the transformer assembly. Commonly, however, a certain degree of variation in the bias circuit remains. Additionally, a particular application may require a fractional turns adjustment, which is often costly and inefficient to implement.
The regulation of the bias circuit voltage usually takes into account an adjustment for input line and output load, which again, results in large variation. Again, regulation by turns ratio adjustment can result in extra cost and lost efficiency. For example, 100-200 mA of current at a 10-12V level may be desired for a particular application. The primary side of the transformer assembly can vary between 36 and 75 volts. In a typical situation, the resulting regulated bias circuit voltage can vary between 10 and 20 volts. Because only 10 volts are desired for the application, the extra 10 volts of headroom or approximately 1 to 2 watts (10V*100 to 200 mA) of power is dissipated as heat.
In addition, adjustment of turns ratios to regulate bias voltages can take up valuable space which could be devoted for other circuit components or to make the footprint of the power supply smaller.
A need exists to supply a relatively constant bias circuit voltage which can be more effectively and efficiently regulated than traditional turns ratio adjustment of transformer assemblies.